1. Field of the Invention
This invention relates to a ceramic package, and more particularly, to such a ceramic package comprising a ceramic base and an improved electroplated metal underlayer formed on the ceramic base.
2. Description of the Related Art
A ceramic package used for a semiconductor device, known in the prior art, comprises a ceramic base having a die-bonding area and wire-bonding areas, and metal layers formed on the die-bonding area and wire-bonding areas of the ceramic base. Such metal layers on the ceramic base are usually made by:
forming on the ceramic base with a metallized layer consisting of tungsten or the like, PA1 usually electroless plating with nickel, PA1 brazing metal leads to the metallized layer on the ceramic base, PA1 electroplating the underlayer with nickel as a surface treatment, and PA1 also electroplating the upper layer with gold.
The nickel-plated layer on the metallized layer is effective for obtaining a good property for brazing the metal leads. The nickel and gold-plated layers, which are provided after the leads are brazed, are effective for obtaining good bonding characteristics or endurable performance in its environments.
Generally speaking, a ceramic package has been used as a highly reliable package for a semiconductor device. However, in a ceramic package having an underlayer as imprising a nickel-plated layer, as mentioned above, there has been some problems in that a semiconductor chip might be peeled off when the package is subjected to an ageing process after die-bonding, or nickel might be diffused to the surface of the gold-plated layer to change a color of the surface of the gold-plated layer when package is heated. Also, in the recent years, semiconductor devices are sometimes used in relatively severe conditions. Thus, the above-mentioned characteristics and other conditions, for instance, a good soldering characteristic, are more strongly required.
Particularly important is a moisture endurability in the metal layers on the ceramic base and thus a ceramic package having a high moisture endurability is required, since the metal layers might become rusty in a moisture environment, giving rise to problems, such as an electric short circuit between conductive patterns.
In the prior art, to overcome the above-mentioned problems, Japanese Unexamined Patent Publication (Kokai) No. 59-155950 suggests that a cobalt-plated layer is used as an underlayer, Japanese Examined Patent Publication (Kokoku) No. 63-3036 suggests that a cobalt-nickel alloy is applied as an underlayer, and also Unexamined Patent Publication (Kokai) No. 63-83291 suggests that an alloy of cobalt, sulfur, and nickel is used as an underlayer to overcome the above-mentioned problems.
However, if a cobalt-plated layer were used as an underlayer, the cobalt might readily be oxidized to reduce the anti-corrosion characteristic and, therefore, the color of the package might readily be changed.
In a ceramic package, in which cobalt-nickel alloy is used as an underlayer, if the cobalt ratio in the alloy were low, the die bondability would poor and also discoloration of the upper gold-plated layer might be occur depending on the die bonding temperature. On the other hand, if the cobalt ratio in alloy were high, the discoloration characteristic and corrosion resistance would be deteriorated.
Also, as compared with an underlayer of nickel or cobalt only, an underlayer of cobalt-nickel alloy is disadvantageous in uniformness of the electroplated film thickness, because the throwing power of the alloy plating bath is poorer than that of the nickel or cobalt plating bath. Controling the plating bath is also difficult to preserve a certain quality and compositions thereof.